High temperature superalloys such as cobalt and nickel-based superalloys are widely used to manufacture certain components of gas turbine engines, including combustors and turbine vanes and blades. While high temperature superalloy components are often formed by casting, circumstances exist where such components are preferably or required to be fabricated by brazing. For example, components having complex configurations can be more readily fabricated by brazing or welding separate subcomponents together, such as the turbine blades and vanes of a gas turbine engine. Therefore, it is typically more practical and cost effective to fabricate complex components by brazing or welding rather than casting the component as a single member.
In addition, brazing and welding are widely used as a method for repairing cracks and other surface discontinuities that result from thermal cycling or foreign object impact of a superalloy component. Because the cost of components formed from high temperature cobalt and nickel-based superalloys is relatively high, repairing these components is typically more desirable than replacing them when they become damaged.
In the prior art, brazing methods for superalloy components have included vacuum brazing techniques using alloy powders or mixtures of powders. With the advent of higher strength and more highly alloyed superalloys, improved braze materials and processes have been required to take full benefit of the strength of such superalloys. Because the mechanical properties of a braze material generally increase with higher melting temperatures, a significant effort has been directed toward formulating braze materials whose melting points approach that of superalloys.
To promote a high quality braze joint during component fabrication or repair, the surfaces to be joined must be cleaned with the use of abrasive media to remove oxides and contamination that would otherwise reduce the strength of the bond. This mode of cleaning inherently introduces cold work into the surface of the superalloy, as do numerous other processes conventionally performed on superalloy articles, such as welding, stripping and recoating, grinding and milling. Even handling of an article can result in cold working of the article in the form of localized surface deformation. Any such surface work will cause recrystallization in the surface of the superalloy article if the surface is later subjected to a sufficiently high temperature, such as during brazing or welding. As is known in the art, recrystallization substantially reduces the mechanical properties of superalloys, including thermal fatigue resistance, and provides boundaries where cracking can occur. These adverse effects are particular deleterious for single crystal (SX) and directionally-solidified (DS) components used in gas turbine engines, such as high pressure turbine vanes and blades and combustors.
Therefore, the optimal brazing temperature for brazing a superalloy component is dictated in part by two conflicting considerations. On one hand, braze materials whose melting temperatures approach that of the superalloy provide stronger brazements. However, the temperature at which brazing can be performed is limited by the temperature at which recrystallization will occur in the surface of a superalloy as a result of its surface being cold worked during machining, cleaning and handling. With nickel and cobalt-base superalloys, cellular recrystallization can be of concern at brazing temperatures of as low as about 930.degree. C. (about 1700.degree. F.), imposing a significant limitation on the maximum melting temperature of the braze material used to braze a superalloy article, whose melting temperature often exceeds 1300.degree. C.
Accordingly, it can be appreciated that prevention of recrystallization during heating of a superalloy component would be highly desirable. In particular, such a capability would enable a higher temperature brazing operation using braze materials with melting temperatures that more closely approach that of the superalloy from which the component is formed. Finally, such a capability would be particularly advantageous with single crystal and directionally solidified gas turbine engine components, whose mechanical properties are predominantly dependent on their microstructures.